1. Field of the Invention
The present invention relates to a broadband astigmatic feed arrangement for an antenna and, more particularly, to a broadband astigmatic feed arrangement comprising a first and a second doubly curved subreflector which are curved in orthogonal planes to permit the launching of an astigmatic beam of constant size and shape over a broadband frequency range. Additionally, the proper angular interpositioning of both reflectors can enable substantial elimination of cross-polarization coupling.
2. Description of the Prior Art
Except for possibly the axial beam of an antenna, reflectors generally will introduce some sort of aberration if the feedhorn is located away from the geometrical focus. Consequently, the wavefront of an off-axis beam is not planar. This is especially true in a multibeam reflector antenna system. Antenna systems, however, have been previously devised to correct for certain aberrations which have been found to exist.
U.S. Pat. No. 3,146,451 issued to R. L. Sternberg on Aug. 25, 1964 relates to a microwave dielectric lens for focusing microwave energy emanating from a plurality of off-axis focal points into respective collimated beams angularly oriented relative to the lens axis. In this regard also see U.S. Pat. No. 3,737,909 issued to H. E. Bartlett et al. on June 5, 1973.
U.S. Pat. No. 3,569,795 issued to G. C. Fretz, Jr. on Mar. 9, 1971 relates to apparatus for altering an electromagnetic wave phase configuration to a predetermined nonplanar front to compensate for radome phase distortion and which wave, upon exiting the radome, has a phase front which is planar.
Other antenna system arrangements are known which use subreflectors and the positioning of feedhorns to compensate for aberrations normally produced by such antenna systems. In this regard see, for instance U.S. Pat. Nos. 3,688,311 issued to J. Salmon on Aug. 29, 1972; 3,792,480 issued to R. Graham on Feb. 12, 1974; and 3,821,746 issued to M. Mizusawa et al. on June 28, 1974.
U.S. Pat. No. 3,828,352 issued to S. Drabowitch et al. on Aug. 6, 1974 relates to microwave antennas including a toroidal reflector designed to reduce spherical aberrations. The patented antenna structure comprises a first and a second toroidal reflector centered on a common axis of rotation, each reflector having a surface which is concave toward that common axis and has a vertex located in a common equatorial plane perpendicular thereto.
U.S. Pat. No. 3,922,682 issued to G. Hyde on Nov. 25, 1975 relates to an aberration correcting subreflector for a toroidal reflector antenna. More particularly, an aberration correcting subreflector has a specific shape which depends on the specific geometry of the main toroidal reflector. The actual design is achieved by computing points for the surface of the subreflector such that all rays focus at a single point and that all pathlengths from a reference plane to the point of focus are constant and equal to a desired reference pathlength. The Hyde subreflector, however, (a) only corrects for on-axis aberration of the torus (similar to spherical aberration), (b) only compensates for aberrations when positioned in the far field of the feed, and (c) can be used to produce offset beams in only one plane.
U.S. Pat. No. 4,145,695 issued to M. J. Gans on Mar. 20, 1979 relates to launcher reflectors which are used with reflector antenna systems to compensate for the aberration of astigmatism which was found to be introduced in the signals being radiated and/or received at the off-axis positions. A major portion of such phase error is corrected by using, with each off-axis feedhorn, an astigmatic launcher reflector having a curvature and orientation of its two orthogonal principal planes of curvature which are chosen in accordance with specific relationships, the launcher reflector being fed by a symmetrical feedhorn.
Prior art arrangements, however, have only compensated for astigmatism introduced by off-axis position of a reflector over a certain band of frequencies. The problem, therefore, remaining is to provide feed arrangements for the correction of astigmatism in off-axis fed reflector antennas over a broad band of frequencies.